For one, it would be really hard to replace fiat currencies despite the fact that it is basically inferior in functionality and value than bitcoins or cryptocurrency for that matter. There are some commodities that would need regulation from the government or else it would cause a collapse in the society. One of these is money. If we have a money we cannot regulate, it's gonna cause a problem to most people.
Despite the success records of Bitcoins and individuals' investment in bitcoins, Bitcoin may not replace the traditional currencies. As traditional currencies are printed & controlled by the governments whereas bitcoin doesn't control by a third party.
No doubt, investors and individuals considering it as a safe vehicle against the finical crisis. But, many people still don't know about what is bitcoin, how it works, and the investment benefits of bitcoins.
Added to the fact that they don't know bitcoin is that they know what bitcoin isn't. People tend to retaliate when learning that some things have shortcomings on them, and without knowing the benefits yet, they will have a problem accepting bitcoin.